Although it turned out that Wegener was right about mobile continents, his theory was heavily criticized and did not immediately settle the controversy over the Permo-Carboniferous glaciation. Wegener was quite selective in his choice of evidence for continental drift, and he simply ignored things that seemed contradictory. He also could not identify a satisfactory mechanism for moving the continents about—his one suggestion, that centrifugal forces associated with the Earth’s rotation might be responsible, was quickly demolished by his opponents. As a result of these difficulties, the theory was generally discounted, and with it the idea that the Permo-Carboniferous glaciation occurred when the southern continents were joined together and located near the South Pole. It was not until almost half a century later that the concept of continental drift was resurrected and transformed into the modern theory of plate tectonics. But present-day reconstructions of the southern continents at the time of the Permo-Carboniferous glaciation show that Wegener’s conclusion was essentially correct. Africa and South America had been joined together like pieces of a jigsaw puzzle, with India snuggled up against Madagascar on the east coast of Africa. The Antarctic continent and Australia were firmly attached and nestled along the southern tips of Africa, South America, and India. This entire supercontinent, referred to by geologists as Gondwanaland, had been centered near the South Pole throughout the long Permo-Carboniferous Ice Age. When locations of the glacial deposits are plotted on the fused-together continent, they reveal the former presence of a single large ice sheet (figure 18).
The development of the theory of plate tectonics also solved a related puzzle. Glacial markings often reveal the direction of ice flow, and some of the striations and scratches left by the Permo-Carboniferous glaciation indicate that the ice flowed from the sea onto the land, at least in terms of the present-day configuration. In India, for example, they show ice moving inland from the Arabian Sea. The evidence is unambiguous, but because ice flows downhill under gravity, the conclusion appeared to early geologists to be impossible. Glaciers form on land and flow into the sea; how could great thicknesses of ice build up in the ocean and spread onto the land? But with the continents joined together as Gondwanaland, there were no intervening seas. Ice simply flowed outward from its thick center across the present-day continental boundaries, creating the impression—after the continents were separated—that it had flowed inland from the oceans.
The most reliable dating of the Permo-Carboniferous Ice Age suggests that its maximum extent occurred during a span of about 20 million years, from approximately 280 to 300million years ago, and that ice sheets extended over the supercontinent of Gondwanaland to at least 40° south latitude, and perhaps even to 35° S or less. That equals or exceeds the maximum spread of the Pleistocene glaciers in the Northern Hemisphere during the current ice age, and it is reasonable to infer that the volume of ice at the height of the Permo-Carboniferous glaciation exceeded the maximum, so far, of the Pleistocene glaciation.
But how do we know this? How can the latitudes of, say, Buenos Aires or Cape Town three hundred million years ago be determined, when we know the continents have continuously moved about on the Earth’s surface? The relative positions of the continents can be worked out quite far back into the past, but locating them with respect to the poles is a more difficult problem. And yet estimating the extent of ice sheets, particularly estimating how closely they approached the equator, is crucial for determining the severity of an ancient ice age. Fortunately, the Earth’s magnetic field provides a tool for this problem, just as it does for the timescale of deep-sea sediments. The field is very close to one that would be produced if there were a gigantic bar magnet inside the Earth. Viewed from space, it would look just like the textbook example of iron filings sprinkled around a bar magnet: the field lines form great arcs that join the magnet at its top and bottom and bulge outward at its sides. And at every latitude, the field has a specific orientation relative to the Earth’s surface, ranging from 90° at the poles to 0° at the equator.
As was discussed in the previous chapter, some minerals line up their internal magnetic fields with the Earth’s when they form, just as the iron filings line up with the field of a bar magnet. And because the orientation of the Earth’s field depends on latitude, such minerals essentially encode the latitude in their physical properties at the time of their formation or when they are incorporated into a sedimentary layer. In favorable cases, the magnetic information can be deciphered in the laboratory, and although it’s not always possible to tell which hemisphere the rock formed in (the inclination of the magnetic field to the surface is the same at equivalent latitudes both north and south of the equator), it is possible to determine the critical parameter for glaciation, how close it was to the equator. Like most geological records, the evidence from rock magnetism gets more and more fragmentary as one delves farther and farther back into geological time, but for the Permo-Carboniferous glaciation, the data are abundant. Not only can the positions of the continents that made up Gondwanaland be located quite precisely, but it’s also possible to reconstruct how the supercontinent drifted slowly across the South Pole during the long ice age. The record left by the ice itself is fully consistent with the magnetic evidence, for it shows that the center of the ice sheets—the region from which ice flowed outward in all directions—stayed roughly fixed in latitude near the pole as the continent drifted slowly through the region.
Today’s northern continents seem to have largely escaped the Permo-Carboniferous glaciation. This too is consistent with the magnetic evidence, which indicates that they were not then located in polar regions. Only parts of what is now Siberia extended to high northern latitudes; North America and Europe were farther south. There are some signs of localized glaciers in Siberia, but in Europe and North America, the tillites, glacially scratched rocks, and other ice effects that exist in the former Gondwanaland are absent. But there is one striking feature of the geologic record in these regions that has been linked, indirectly, to glaciation: an abundance of coal deposits.
The Carboniferous period derives its name from the widespread carbon-rich deposits that occur in this interval of geologic time. They are mostly made up of the remains of spore-bearing plants similar to ferns, plants that lived in low-lying, moist environments often referred to as coal swamps. The organic debris that accumulated in these swamps formed peat deposits; these in turn were eventually transformed into coal. A peculiar feature of the Carboniferous coal deposits in North America and Western Europe is that they are cyclical: beds of coal alternate with marine sedimentary rocks such as limestone or shale in a pattern that is repeated many times over. In places as many as a hundred cycles occur; although it is difficult to determine the amount of time represented by each cycle, in aggregate it is estimated that they span ten million years of deposition, or even more. The plants that were the precursors of the coal grew in fresh or slightly brackish water, and it is believed that many of the coal deposits formed in low-lying coastal swamps that were periodically inundated with seawater. With each flooding the accumulated peat was buried beneath a layer of ocean sediment; between floodings the fresh water swamps reestablished themselves and new layers of peat accumulated. The pressure and heat of burial eventually transformed the multiple peat layers into the cyclical coal deposits that characterize the Carboniferous.
The link between coal deposits and the Permo-Carboniferous Ice Age has to do with the repeated flooding of the coal swamps, which was most likely due to rising and falling sea level. All glacial ice has its source in the ocean through the Earth’s ongoing cycle of evaporation and precipitation; if all the water currently frozen in the Greenland and Antarctic ice caps were returned to the ocean, sea level would rise by about 60 meters, more than enough to drown a coastal swamp. At the height of the Permo-Carboniferous glaciation, the amount of water locked up in the ice sheets was probably equivalent to somewhere between 150 and 250 meters of ocean depth. Even if only a fraction of these ice sheets melted a
nd then reaccumulated in glacial-interglacial cycles similar to those of the Pleistocene Ice Age, the resulting changes in sea level would be quite sufficient to explain the cyclical Carboniferous coal deposits. The cause of the alternating warm-cold periods of the Permo-Carboniferous Ice Age is unknown, but their regularity hints at an astronomical or other external “pacemaker,” just as it did for the current ice age.
The Permo-Carboniferous Ice Age was long and severe, with vast regions of the southern continents buried under ice that extended from the South Pole to low latitudes. Although life on Earth was not nearly as diverse 300 million years ago as it is now, the existing fossil record shows that it was significantly affected. Throughout the Gondwanaland supercontinent, the diversity of plant life dwindled, and the species that survived were hardy varieties adapted to harsh climatic conditions. But as severe as the Permo-Carboniferous glaciation was, it has gradually become apparent that there was an earlier period in the Earth’s history that was even worse. Although there is lively debate about just how much worse it really was, there is general agreement that it was probably the most pervasively cold era in our planet’s history. Some researchers contend that the entire planet was frozen—not only was there ice on the continents, but the ocean surface was frozen as well. This has been called the Snowball Earth hypothesis. Those who opt for a slightly less extreme climate refer to Slushball Earth. Regardless, it was intensely cold, much colder than the Earth has ever been in human experience.
Snowball Earth occurred about 300 million years before the Permo-Carboniferous glaciation, during the long interval between 550 and 850 million years ago. Several separate ice ages may have occurred during this period, but the uncertainties in dating glacial deposits and the difficulty of correlating from continent to continent mean that the entire interval is usually referred to as a single ice age. It occurred near the end of the Proterozoic eon of the geological timescale, and to distinguish it from other icy episodes, geologists refer to it as the Late Proterozoic glaciation. The Late Proterozoic Earth was a very different world than the one we know today—plants and animals had not yet appeared on land, and only relatively primitive life inhabited the sea. The continents were mostly barren rock, the oceans contained no fish or lobsters or seaweed, and there is good evidence that even the atmosphere was quite different, with much less oxygen than at present. Because the Late Proterozoic Ice Age happened so long ago, the evidence of glaciation, although very strong, is fragmentary. It remains only in places where it could be easily preserved, usually places that were already submerged at the time of glaciation, or were low-lying and later flooded by the sea as the glaciers receded. In such environments, glacial drift, scratched and scoured bedrock, and other glacial features were buried under sediments and stored for hundreds of millions of years—and later uplifted again for geological inspection today. But in spite of the rather stringent conditions required for its preservation, the evidence for glaciation during the Late Proterozoic is widespread. It is found on every continent, suggesting that ice sheets were present throughout the globe.
The widespread distribution of glacial features has long convinced geologists that this ice age was unusually harsh. At least two and perhaps as many as four or five major icy episodes, separated by warmer intervals, have been identified during the long cold period. But the big surprise about the Late Proterozoic glaciation came in the late 1950s and early 1960s, when researchers found that the magnetic properties of rocks associated with some of the glacial deposits indicated formation at very low latitudes. As we have seen, during the present ice age, Northern Hemisphere glaciers have never pushed farther south than 40–45°, and even for the more severe Permo-Carboniferous Ice Age, there is no indication that ice reached much closer to the equator than 35° latitude. In both cases, fossil evidence suggests that the tropics remained fairly warm. If Late Proterozoic glaciers had existed near sea level in the tropics, that would indicate a very different ice age indeed, and initially many geologists were skeptical. But as more and more data became available, the initial results were corroborated. It seemed inescapable that frigid climates had extended very close to the equator. Thus was born the concept of Snowball Earth.
The idea that there was a truly global ice age, a Snowball Earth, is still controversial. Geological evidence suggests that during most of our planet’s history, the Earth’s average temperature has stayed within rather narrow limits, and critics ask, Under what conditions could this have changed so that the Earth froze over completely, from pole to equator? And, if it happened once, why haven’t we had more Snowball Earth episodes? At the extreme of Snowball Earth conditions, the average surface temperature would have been closer to that of Mars than anything we are familiar with today—perhaps about –50°C. Could even the primitive life of the Late Proterozoic have survived such extremes? There is general agreement that the glaciation was severe, but as this is written, the jury is still out on whether or not a true Snowball Earth occurred. It is nonetheless worth examining some of the major issues in the controversy.
The magnetic data are perhaps most crucial for the hypothesis, because they fix the latitude of the continents at the time of glaciation. The term “Snowball Earth” itself was coined because magnetic measurements placed the glaciated regions in the tropics, so it is reasonable to assume that their reliability has been carefully scrutinized. It is well known that magnetic measurements can be problematic, especially for old rocks, because they rely on a very accurate determination of the “frozen in” magnetic orientation that was captured when the rocks formed. With care, the orientation can be measured quite accurately with respect to the rock’s current position on the Earth’s surface, but what happens if the rocks have been folded, or tilted, or otherwise moved from their original positions at some time over the past 700 million years? And what if they have been deeply buried and heated, as often happens? Heating can have a significant effect on the stored magnetism, and in extreme cases, heated rocks can be remagnetized—perhaps in an orientation that is completely different from the original. Fortunately, there are ways around these problems. Those who study the magnetic properties of rocks have devised ingenious laboratory approaches that allow them to strip away the magnetic “overprinting” caused by heating and to recover the original magnetic orientation. And because the layers of sedimentary rocks are always horizontal when they are first deposited, folded and tilted sedimentary rocks can be “unfolded” or “untilted” (not literally, but by applying a correction factor to the measured data) to recover their original orientation. Doing this for several samples tilted or folded at different angles actually provides a very good test of the reliability of the measurements: if they all agree after unfolding, one can have a high degree of confidence in the results.
Magnetic measurements have been made on rocks from around the world that are associated with the Late Proterozoic glaciation. They have been repeated in different laboratories with good agreement, and they show that the majority of these localities were at latitudes less than 30° at the time of glaciation, and none were farther than 60° from the equator. In fact, there seem to have been no landmasses near either of the poles in the Late Proterozoic.
Even if glaciers existed at low latitudes, there is still the question of whether the evidence we have comes from ice in high mountains, or at low elevations. This is not such a difficult question to answer, because, as already mentioned, most of the glacial effects that remain from the Late Proterozoic are preserved in sedimentary rocks. It is fairly clear that some of the tillites were either deposited in shallow seawater at the edge of a continent, or so close to the shoreline that a slight change in sea level engulfed them in marine sediments. The same is true of preserved glacial markings on bedrock. Overall, the evidence is very strong that even the tropical ice sheets extended right down to sea level.
However, even such extreme climate conditions do not automatically lead to the central conclusion of the Snowball Earth theory: that the oceans were frozen t
oo. Evidence for that idea came first from an examination of ocean sediments from the Late Proterozoic by Joe Kirschvink, a geochemist at CalTech, who coined the term “Snowball Earth.” In 1992, Kirschvink pointed out that peculiar sedimentary deposits rich in iron, referred to as banded iron formations, or BIFs, occur in a number of localities around the world just at the time of the Late Proterozoic glaciation. Geologists were familiar with BIF deposits from very early in the Earth’s history, but none were known for about a billion years before the Late Proterozoic glaciation, and none have formed since that time. Their occurrence requires the buildup of very large amounts of dissolved iron in seawater, a phenomenon that cannot occur today because of the oxygen-rich atmosphere. As anyone who has had to deal with rusty metal knows only too well, oxygen combines rapidly with iron and forms rust. BIFs are basically rust deposits (with a few other components as well), and their restriction to the early part of the geologic record is thought to be due to the low concentration of oxygen in the atmosphere at that time. Under such conditions, iron from the weathering of both continental and undersea rocks would accumulate in the oceans until it came into contact with oxygen—perhaps produced by photosynthetic algae living in surface waters—whereupon it would be oxidized and precipitate out as a BIF. The occurrence of BIFs in the Late Proterozoic was an enigma, because by that time in the Earth’s history, there was enough atmospheric oxygen to prevent the necessary buildup of dissolved iron in the ocean. But Kirschvink reasoned that if the ocean were frozen, preventing any exchange with the atmosphere, its oxygen content would be rapidly depleted. Iron concentrations would increase to high levels, and BIFs would be deposited when the sea ice melted and oxygen from the atmosphere again began to exchange with the ocean.
Kirschvink’s proposal seemed reasonable, but it was not convincing to everyone. Perhaps locally oxygen-poor basins—which occur because of restricted circulation even in today’s oceans—could have served as hosts for BIFs during Snowball Earth. That would still not explain why they are absent before and after the Late Proterozoic, but it did cast some doubt on the frozen ocean hypothesis. Then, in 1998, four Harvard University researchers, led by the geologist Paul Hoffman, published the results of a study they had made in northern Namibia, which, they believed, made a strong case for the Snowball Earth theory. The region showed clear evidence of low latitude (approximately 12°S) glaciation between about 760 and 700 million years ago. An interesting and important aspect of their work was that the rock sequence they investigated indicated that the glaciation had both started and ended abruptly. Immediately overlying the glacial deposits—as is the case at many other Late Proterozoic Ice Age localities—they found limestone-like sedimentary rocks of a kind that form only in warm, tropical waters. Where they occur over glacial deposits, these distinctive formations have been termed “cap carbonates” by geologists. They signify a rapid transition from very cold to very warm conditions.
Frozen Earth: The Once and Future Story of Ice Ages Page 18
